Electro-optical liquid-crystal display

ABSTRACT

The invention relates to an electro-optical liquid-crystal display having a realignment layer for realigning the liquid crystals, where the field thereof has a significant component parallel to the liquid-crystal layer, comprising a liquid-crystalline medium having negative dielectric anisotropy, where the medium comprises at least one mesogenic compound which contains at least four compounds which contain a group of the formula A

SUMMARY OF THE INVENTION

The invention relates to an electro-optical liquid-crystal displayhaving a realignment layer for realigning the liquid crystals, where thefield thereof has a significant component parallel to the liquid-crystallayer, comprising a liquid-crystalline medium having negative dielectricanisotropy, where the medium comprises at least four mesogenic compoundswhich contain a group of the formula A

In conventional liquid-crystal displays {TN, STN, OMI and AMD-TN}, theelectric fields for realignment are produced essentially perpendicularto the liquid-crystal layer.

International Patent Application WO 91/10936 discloses a liquid-crystaldisplay in which the electric signals are produced in such a way thatthe electric fields have a significant component parallel to theliquid-crystal layer (IPS, in-plane switching). The principles ofoperating such a display are described, for example, by R. A Soref inJournal of Applied Physics, Vol. 45, No. 12, pp. 5466-5468 (1974).

EP 0 588 568 discloses various ways of addressing such a display.

These IPS displays can be operated with liquid-crystalline materialseither having a positive dielectric anisotropy or having a negativedielectric anisotropy (Δε≠0). The specification describes, inter alia,an IPS display containing the mixture ZLI-2806 (E. Merck, Darmstadt)having negative dielectric anisotropy.

This mixture is based on compounds which contain a structural unit ofthe formula A However, IPS displays containing this material haverelatively high threshold voltages and long response times. The objectwas therefore to indicate liquid-crystalline materials which aresuitable for achieving relatively low threshold voltages and shortresponse times in IPS displays.

Surprisingly, this object has been achieved by using liquid-crystallinematerials which comprise at least four compounds which contain a groupof the formula A

Such compounds are disclosed, for example, in EP 0 332 007 (formula A).

However, these specifications give no indication that these substancescan be used to improve the threshold voltages and the response times ofIPS displays.

The invention thus relates to an electro-optical liquid-crystal displayhaving a realignment layer for realigning the liquid crystals, where thefield thereof has a significant component parallel to the liquid-crystallayer, e.g., a parallel compound strong enough to force the switching ofthe molecules in the plane compared to the switching out of the planeas, e.g., in TN and STN or into the plane, as, e.g., in ECB, comprisinga liquid- crystalline medium having negative dielectric anisotropy,where the medium comprises at least four mesogenic compounds whichcontain a group of the formula A

Preferred embodiments are IPS displays in which (a) the medium comprisesat least one compound of the formula I

in each of which

R and R^(a) are each an alkyl or alkenyl radical having 1 to 15 carbonatoms which is unsubstituted, monosubstituted by CN or CF₃ or at leastmonosubstituted by halogen, where, in addition, one or more CH₂ groupsin these radicals may each, independently of one another, be replaced by

in such a way that O atoms are not linked directly to one another,

A¹ and A² are each, independently of one another,

(a) a trans-1,4-cyclohexylene radical in which, in addition, one or morenon-adjacent CH₂ groups may be replaced by —O— and/or —S—,

(b) a 1,4-phenylene radical in which, in addition, one or two CH groupsmay be replaced by N,

(c) a radical from the group consisting of 1,4-cyclohexenylene,1,4-bicyclo-[2.2.2]octylene, piperidine- 1,4-diyl, naphthalene-2,6-diyl,decahydro-naphthal- ene-2,6-diyl and1,2,3,4-tetra-hydronaphthalene-2,6-diyl,

where the radicals (a) and (b) may be substituted by one or two fluorineatoms,

Z¹ and Z² are each, independently of one another, —CO—O—, —O—CO—,—CH₂O—, —OCH₂—, —CH₂CH₂—, —CH═CH—, —C═C— or a single bond, or one of theradicals Z¹ and Z² is alternatively —(CH₂)₄— or —CH═CH—CH₂CH₂—,

m and n are 0, 1 or 2 and

m+n is 1, 2 or 3;

b) the medium has a dielectric anisotropy Δε of <−4.6, preferablybetween −4.7 and −8; and/or

c) the medium comprises at least one compound of the formula II

R³-(A³-Z³)_(o)-A⁴-R⁴  II

in which

R³ and R⁴ each, independently of one another, are as defined for R¹ andR^(a),

A³ and A⁴ each, independently of one another, are as defined for A¹ andA²,

Z³ in each case independently of the others, is as defined for Z¹ andZ², and

o is 1, 2 or 3.

Preference is even more given to an IPS display in which the pixels areaddressed by means of an active matrix, and to mixtures wherein the fourcompounds of (A) are compounds of formula I.

The invention furthermore relates to a liquid-crystalline medium havingnegative dielectric anisotropy which comprises at least one compound ofthe formula I and at least one compound of the formula II, in particularwhich comprises

from 15 to 85 % by weight, preferably from 30 to 75 % by weight, of atleast four compounds of the formula I, and

from 10 to 55 % by weight, preferably from 10 to 40 % by weight, of atleast one compound of the formula II.

The novel liquid crystalline medium preferably comprises

at least one compound of the formula Ia, and

at least one compound of the formula IIb

at least one compound selected from the formulae IIa, IIb and IIc

in which

and R, R^(a), R³ and R⁴ are in each case as defined above.

The novel liquid-crystalline media generally have a birefrigence (Δn) of<0.10, preferably between 0.04 and 0.09, in particular between 0.05 and0.07.

The bulk viscosity (at 20° C.) of the novel materials is generally lessthan 30 mm² s⁻¹, in particular between 15 and 25 mm²·s⁻¹. Theresistivity of the novel materials is generally between 5×10¹⁰ and7×10¹³ Ω·cm⁻¹ at 20° C.

It has been found that even a relatively small proportion of compoundsof the formula I mixed with conventional liquid-crystal materials, butin particular with one or more compounds of the formula II, results in asignificant reduction in the threshold voltage and in fast responsetimes, and at the same time broad nematic phases having lowsmectic-nematic transition temperatures are observed. The compounds ofthe formulae I and II are colorless, stable and readily miscible withone another and with other liquid-crystal materials.

The term “alkyl” covers straight-chain and branched alkyl groups having1-7 carbon atoms, in particular the straight-chain groups methyl, ethyl,propyl, butyl, pentyl, hexyl and heptyl. Groups having 2-5 carbon atomsare generally preferred.

The term “alkenyl” covers straight-chain and branched alkenyl groupshaving 2-7 carbon atoms, in particular the straight-chain groups.Particular alkenyl groups are C₂-C₇-1E-alkenyl, C₄-C₇3E-alkenyl,C₅-C₇-4-alkenyl, C₆-C₇-5-alkenyl and C₇-6-alkenyl, in particularC₂-C₇-1E-alkenyl, C₄-C₇-3E-alkenyl and C₅-C₇-4-alkenyl. Examples ofpreferred alkenyl groups are vinyl, 1E-propenyl, 1E-butenyl,1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl, 3E-pentenyl,3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl, 4E-hexenyl,4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groups having up to 5carbon atoms are generally preferred.

The term “fluoroalkyl” preferably covers straight-chain groupscontaining terminal fluorine, i.e. fluoromethyl, 2-fluoroethyl,3-fluoropropyl, 4-fluorobutyl, 5-fluoropentyl, 6-fluorohexyl and7-fluoroheptyl. However, other positions of the fluorine are notexcluded.

The term “oxaalkyl” preferably covers straight-chain radicals of theformula C_(n)H_(2n+1)—O—(CH₂)_(m), in which n and m are each,independently of one another, from 1 to 6. n is preferably 1 and m ispreferably from 1 to 6.

Through a suitable choice of the meanings of R, R^(a), R³ and R⁴, theaddressing times, the threshold voltage, the steepness of thetransmission characteristic lines, etc., can be modified as desired. Forexample, I E- alkenyl radicals, 3E-alkenyl radicals, 2E-alkenyloxyradicals and the like generally result in shorter addressing times,improved nematic tendencies and a higher ratio between the elasticconstants k₃₃ (bend) and k₁₁ (splay) compared with alkyl and alkoxyradicals. 4-alkenyl radicals, 3-alkenyl radicals and the like generallygive lower threshold voltages and smaller values of k₃₃/k₁₁ comparedwith alkyl and alkoxy radicals.

The optimum mixing ratio between the compounds of the formulae I and IIdepends substantially on the desired properties, on the choice of thecomponents of the formulae I, and II and on the choice of furtheroptional components. Suitable mixing ratios within the above range caneasily be determined from case to case.

The total amount of compounds of the formulae I and II in the novelmixtures is not crucial. The mixtures can therefore contain one or morefurther components in order to optimise the various properties. However,the observed effect on the addressing times and on the threshold voltageis generally greater the higher the total concentration of compounds ofthe formulae I and II.

The liquid-crystalline media according to the invention preferablycontain 2 to 40, in particular 4 to 30, components as furtherconstituents besides compounds of the formulae I and II. These mediavery particularly preferably contain 7 to 25 components besides thecompounds of formula I according to the invention. These furtherconstituents are preferably selected from nematic or nematogenic(monotropic or isotropic) substances, in particular substances from theclasses of the azoxybenzenes, benzylideneanilines, biphenyls,terphenyls, phenyl or cyclohexyl benzoates, phenyl or cyclohexyl estersof cyclohexane-carboxylic acid, phenyl or cyclohexyl esters ofcyclohexylbenzoic acid, phenyl or cyclohexyl esters ofcyclohexylcyclohexanecarboxylic acid, cyclohexylphenyl esters of benzoicacid, of cyclohexanecarboxylic acid and ofcyclohexylcyclohexanecarboxylic acid, phenylcyclohexanes,cyclohexylbiphenyls, phenylcyclohexyl-cyclohexanes,cyclohexylcyclohexanes, cyclohexylcyclohexylcyclohexenes,1,4-bis-cyclohexylbenzenes, 4,4′-bis-cyclohexylbiphenyls, phenyl- orcyclohexylpyrimidines, phenyl- or cyclohexylpyridines, phenyl- orcyclohexyldioxanes, phenyl- or cyclohexyl- 1,3-dithianes,1,2-diphenylethanes, 1,2-dicyclohexylethanes,1-phenyl-2-cyclohexylethanes,1-cyclohexyl-2-(4-phenylcyclohexyl)ethanes,1-cyclohexyl-2-biphenylylethanes, 1-phenyl-2-cyclohexylphenylethanesoptionally halogenated stilbenes, benzyl phenyl ethers, tolans andsubstituted cinnamic acids. The 1,4-phenylene groups in these compoundsmay also be fluorinated.

The most important compounds suitable as further constituents of mediaaccording to the invention can be characterized by the formulae 1, 2, 3,4 and 5:

R′—L—E—R″  1

R′—L—COO—E—R″  2

R′—L—OOC—E—R″  3

R′—L—CH₂CH₂—E—R″  4

R′—L—C≡C—E—R″  5

In the formulae 1, 2, 3, 4 and 5, L and E, which may be identical ordifferent, are in each case, independently of one another, a bivalentradical from the group formed by -Phe-, -Cyc-, -Phe-Phe-, -Phe-Cyc-,-Cyc-Cyc-, -Pyr-, -Dio-, -G-Phe- and -G-Cyc- and their mirror images,where Phe is unsubstituted or fluorine-substituted 1,4-phenylene, Cyc istrans-1,4-cyclohexylene or 1,4-cyclohexylene, Pyr is pyrimidine-2,5-diylor pyridine-2,5-diyl, Dio is 1,3-dioxane-2,5-diyl and G is2-(trans-1,4-cyclohexyl)ethyl, pyrimidine-2,5-diyl, pyridine-2,5-diyl or1,3-dioxane-2,5-diyl.

One of the radicals L and E is preferably Cyc, Phe or Pyr. E ispreferably Cyc, Phe or Phe-Cyc. The media according to the inventionpreferably contain one or more components selected from the compounds ofthe formulae 1, 2, 3, 4 and 5 in which L and E are selected from thegroup comprising Cyc, Phe and Pyr and simultaneously one or morecomponents selected from the compounds of the formulae 1, 2, 3, 4 and 5in which one of the radicals L and E is selected from the groupcomprising Cyc, Phe and Pyr and the other radical is selected from thegroup comprising -Phe-Phe-, -Phe-Cyc-, -Cyc-Cyc-, -G-Phe- and -G-Cyc-,and optionally one or more components selected from the compounds of theformulae 1, 2, 3, 4 and 5 in which the radicals L and E are selectedfrom the group comprising -Phe-Cyc-, -Cyc-Cyc-, -G-Phe- and -G-Cyc-.

In a smaller sub-group of the compounds of the formulae 1, 2, 3, 4 and5, R′ and R″ are in each case, independently of one another, alkyl,alkenyl, alkoxy, alkoxyalkyl, alkenyloxy or alkanoyloxy having up to 8carbon atoms. This smaller sub-group is called group A below, and thecompounds are labelled with the sub-formulae 1a, 2a, 3a, 4a and 5a. Inmost of these compounds, R′ and R″ are different from one another, oneof these radicals usually being alkyl, alkenyl, alkoxy or alkoxyalkyl.

In another smaller sub-group of the compounds of the formulae 1, 2, 3, 4and 5 which is known as group B, R″ is —F, —Cl, —NCS or—(O)_(i)CH_(3-(k+l))F_(k)Cl_(l), where i is 0 or 1, and k+l is 1, 2 or3; the compounds in which R″ has this meaning are labelled with thesub-formulae 1b, 2b, 3b, 4b and 5b. Particular preference is given tothose compounds of the sub-formulae 1b, 2b, 3b, 4b and 5b in which R″ is—F, —Cl, —NCS, —CF₃, —OCHF₂ or —OCF₃.

In the compounds of the sub-formulae 1b, 2b, 3b, 4b and 5b, R′ is asdefined for the compounds of the sub-formulae I a-5a and is preferablyalkyl, alkenyl, alkoxy or alkoxyalkyl.

In a further smaller sub-group of the compounds of the formulae 1, 2, 3,4 and 5, R″ is —CN; this sub-group is known as group C below, and thecompounds of this sub-roup are correspondingly described by sub-formulae 1c, 2c, 3c, 4c and 5c. In the compounds of the sub-formulae 1c,2c, 3c, 4c and 5c, R′ is as defined for the compounds of thesub-formulae 1a-5a and is preferably alkyl, alkoxy or alkenyl.

In addition to the preferred compounds of groups A, B and C, othercompounds of the formulae 1, 2, 3, 4 and 5 having other variants of theproposed substituents are also customary. All these substances can beobtained by methods which are known from the literature or analogouslythereto.

Besides compounds of the formula I according to the invention, the mediaaccording to the invention preferably contain one or more compoundsselected from group A and/or group B and/or group C. The proportions byweight of the compounds from these groups in the media according to theinvention are preferably

Group A: 0 to 90%, preferably 20 to 90%, in particular 30 to 90%

Group B: 0 to 80%, preferably 10 to 80%, in particular 10 to 65%

Group C: 0 to 80%, preferably 5 to 80%, in particular 5 to 50%,

the sum of the proportions by weight of the group A and/or B and/or Ccompounds present in the particular media according to the inventionpreferably being 5% to 90% and in particular 10% to 90%.

The media according to the invention preferably comprise 1 to 40%,particularly preferably 5 to 30%, of compounds according to theinvention. Further preferred media are those which comprise more than40%, in particular 45 to 90%, of compounds according to the invention.The media preferably comprise three, four or five compounds according tothe invention.

The structure of the novel IPS display corresponds to the constructionwhich is conventional for such displays, as described, for example, inWO 91/10936 or EP 0 588 568. The term conventional construction here isbroadly drawn and also covers all derivatives and modifications of theIPS display, in particular, for example, also matrix display elementsbased on poly-Si, TFT or MIM.

However, an essential difference between the novel displays and theconventional ones is in the choice of the liquid-crystal parameters ofthe liquid-crystal layer.

The liquid-crystal mixtures which can be used in accordance with theinvention are prepared in a manner which is conventional per se. Ingeneral, the desired amount of the components used in the lesser amountis dissolved in the components making up the principal constituent,expediently at elevated temperature. It is also possible to mixsolutions of the components in an organic solvent, for example inacetone, chloroform or methanol, and, after thorough mixing, to removethe solvent again, for example by distillation.

The dielectrics may also contain further additives known to the personskilled in the art and described in the literature. For example, 0-15 %of pleochroic dyes or chiral dopes can be added.

C denotes a crystalline phase, S denotes a smectic phase, SB denotes asmectic B phase, N denotes a nematic phase and I denotes the isotropicphase.

V₁₀ denotes the voltage for 10% transmission (viewing directionperpendicular to the plate surface). t_(on) denotes the switch-on timeand t_(off) denotes the switch-off time at an operating voltagecorresponding to 2.5 times the value of V₁₀. An denotes the opticalanisotropy and n₀ denotes the refractive index (in each case 589 nm). Δεdenotes the dielectric anisotropy (Δε=ε_(∥)−ε_(⊥), where ε_(∥)denotesthe dielectric constant parallel to the long axes of the molecules andε_(⊥) denotes the dielectric constant perpendicular thereto). Theelectro-optical data were measured in an IPS cell at 20° C., unlessexpressly stated otherwise. The optical data were measured at 20° C.unless expressly stated otherwise.

A planar test cell was used with a initial twist angle of 0° and analignment angle of 0°.

The cells are dark in the “off” state.

In the present application and in the examples below, the structures ofthe liquid-crystal compounds are indicated by acronyms, thetransformation into chemical formulae taking place as in Tables A and Bbelow. All the radicals C_(n)H_(2n+1), are straight-chain alkyl radicalscontaining n carbon atoms. The coding in Table B requires no furtherexplanation. In Table A, only the acronym for the parent structure isgiven. In individual cases, a code follows for the substituents R¹, R²,L¹, L² and L³, separated from the acronym for the parent structure by ahyphen:

Code for R¹, R², L¹, L², L³ R¹ R² L¹ L² L³ nm C_(n)H_(2n+1)C_(m)H_(2m+1) H H H nOm C_(n)H_(2n+1) OC_(m)H_(2m+1) H H H nO.mOC_(n)H_(2n+1) C_(m)H_(2m+1) H H H n C_(n)H_(2n+1) CN H H H nN.FC_(n)H_(2n+1) CN H F H nF C_(n)H_(2n+1) F H H H nOF OC_(n)H_(2n+1) F H HH nCl C_(n)H_(2n+1) Cl H H H nF.F C_(n)H_(2n+1) F H F H nCF₃C_(n)H_(2n+1) CF₃ H H H nOCF₃ C_(n)H_(2n+1) OCF₃ H H H nOCF₂C_(n)H_(2n+1) OCHF₃ H H H nS C_(n)H_(2n+1) NCS H H H rVsNC_(r)H_(2r+1)—CH═CH—C_(s)H_(2s)— CN H H H rEsNC_(r)H_(2r+1)—O—C_(s)H_(2s)— CN H H H nAm C_(n)H_(2n+1) COOC_(m)H_(2m+1)H H H nF.F.F C_(n)H_(2n+1) F H F F nCl.F.F C_(n)H_(2n+1) Cl F F HnCF₃.F.F C_(n)H_(2n+1) CF₃ F F H nOCF₃.F.F C_(n)H_(2n+1) OCF₃ F F HnOCF₂.F.F C_(n)H_(2n+1) OCHF₂ F F H nOCF₃.F C_(n)H_(2n+1) OCF₃ F H HnmFF C_(n)H_(2n+1) C_(m)H_(2m+1) F H F nOmFF C_(n)H_(2n+1) C_(m)H_(2m+1)F H F

Table A and table B contain preferred mixture compounds.

TABLE A

TABLE B

EXAMPLE 1

An IPS display contains a nematic mixture having

clearing point +68° C. Δn [589 nm; 20° C.] 0.0787 n₀ 1.5547 Δε [1 k Hz;20° C.] −3.5 and comprising PCH-502FF 26.40% CCP-302FF 11.44% CCP-502FF10.56% CCP-21FF 8.80% CCP-31FF 8.80% CCH-34 3.52% CCH-501 9.68% CCH-5028.80% PCH-53 12.00%

and has a high contrast and short response times.

EXAMPLE 2

An IPS display contains a nematic mixture having

clearing point +94° C. Δn [589 nm; 20° C.] 0.0836 n₀ 1.5535 Δε [1 k Hz;20° C.] −4.1 and comprising PCH-502FF 23.00% CCP-302FF 15.00% CCP-502FF14.00% CCP-21FF 12.00% CCP-31FF 12.00% CCH-34 4.00% CCH-35 4.00% CCH-5018.00% CCH-502 8.00%

and has a high contrast and short response times.

What is claimed is:
 1. An electro-optical liquid-crystal display havinga realignment layer for realigning the liquid crystals, where the fieldthereof has a significant component parallel to the liquid-crystallayer, comprising a liquid-crystalline medium having negative dielectricanisotropy, wherein the medium comprises at least four mesogeniccompounds of formula I

wherein R and R^(a) are each an alkyl or alkenyl radical having 1 to 15carbon atoms which unsubstituted, monosubstituted by CN or CF₃ or atleast monosubstituted by halogen, where, in one or more CH₂ groups inthese radicals may each, independently of one another, be replaced by

in such a way that O atoms are not linked directly to one another, A¹and A² are each, independently of one another, (a) atrans-1,4-cyclohexylene radical in which, in addition, one or morenon-adjacent CH₂ groups may be replaced by —O—and/or —S—, (b) a1,4-phenylene radical in which, in addition, one or two CH groups may bereplaced by N, (c) a 1,4-cyclohexenylene, 1,4-bicyclo-[2.2.2]octylene,piperidine-1,4-diyl, naphthalene-2,6-diyl,decahydro-naphthalene-2,6-diyl or 1,2,3,4-tetra-hydronaphthalene-2,6-diyl radical, where (a) and (b) may be substitutedby one or two fluorine atoms, Z¹ and Z² are each, independently of oneanother, —CO—O—, —O—CO—, —CH₂O—, —OCH₂—, —CH₂CH_(2—),—CH═CH—, —C≡C— or asingle bond, or one of the radicals Z¹ and Z² is alternatively —(CH₂)₄—or —CH═CH—CH₂CH₂—, m and n are 0, 1 or 2 and m+n is 1, 2 or
 3. at leastone of which is a compound of formulae Ia or Ib


2. A liquid-crystal display according to claim 1, wherein the mediumfurther comprises at least one compound of formula II:R³-(A³-Z³)₀-A⁴-R⁴  II in which R³ and R⁴each, independently of oneanother, are as defined for R and R^(a), A³ and A⁴ each, independentlyof one another, are as defined for A¹ and A², Z³ in each caseindependently is as defined for Z¹ and Z², and o is 1, 2 or
 3. 3. Adisplay according to claim 1, having pixels which are addressed by meansof an active matrix.
 4. A display according to claim 2, wherein theliquid crystalline medium comprises from 15 to 85% by weight of at leastfour compounds of formula I, from 10 to 55% by weight of at least onecompound of the formula II.
 5. A display according to claim 2, whereinthe liquid crystalline medium comprises from 30 to 75% by weight, of atleast four compounds of formula I, from 10 to 40% by weight, of at leastone compound of the formula II.
 6. A display according to claim 2,wherein the liquid-crystlaline medium comprises at least one compound offormula Ia

at least one compound of formula Ib

at least one compound of formulae IIa, IIb or IIc

in which

and R, R^(a), R³ and R⁴ are in each case as defined as above.
 7. Adisplay according to claim 1, wherein the medium comprises at least onecompound of formula Ia.
 8. A display according to claim 1, wherein themedium comprises at least one compound of formula Ib.
 9. A displayaccording to claim 8, wherein the medium comprises at least one compoundof formula Ib.
 10. A display according to claim 1, wherein the mediumfurther comprises at least one compound of the formulae

wherein R is a straight-chain alkyl group, L¹ and L² are F, L³ is H, R²is a straight-chain alkyl group, and R² is an alkoxy group.